相册图片预取缓存策略是内存缓存（硬引用LruCache、软引用SoftReference）、外部文件缓存（context.getCachedDir()），缓存中取不到的情况下再向服务端请求下载图片。同时缓存三张图片（当前预览的这张，前一张以及后一张）。
1.内存缓存
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//需要导入外部jar文件 android-support-v4.jar
import android.support.v4.util.LruCache;
//开辟8M硬缓存空间
private final int hardCachedSize = 8*1024*1024;
//hard cache
private final LruCache sHardBitmapCache = new LruCache(hardCachedSize){
@Override
public int sizeOf(String key, Bitmap value){
return value.getRowBytes() * value.getHeight();
}
@Override
protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue){
Log.v("tag", "hard cache is full , push to soft cache");
//硬引用缓存区满，将一个最不经常使用的oldvalue推入到软引用缓存区
sSoftBitmapCahe.put(key, new SoftReference(oldValue));
}
}
//软引用
private static final int SOFT_CACHE_CAPACITY = 40;
private final static LinkedHashMap> sSoftBitmapCache =
new LinkedHashMao>(SOFT_CACHE_CAPACITY, 0.75f, true){
@Override
public SoftReference put(String key, SoftReference value){
return super.input(key, value);
}
@Override
protected boolean removeEldestEntry(LinkedHashMap.Entry> eldest){
if(size() > SOFT_CACHE_CAPACITY){
Log.v("tag", "Soft Reference limit , purge one");
return true;
}
return false;
}
}
//缓存bitmap
public boolean putBitmap(String key, Bitmap bitmap){
if(bitmap != null){
synchronized(sHardBitmapCache){
sHardBitmapCache.put(key, bitmap);
}
return true;
}
return false;
}
//从缓存中获取bitmap
public Bitmap getBitmap(String key){
synchronized(sHardBitmapCache){
final Bitmap bitmap = sHardBitmapCache.get(key);
if(bitmap != null)
return bitmap;
}
//硬引用缓存区间中读取失败，从软引用缓存区间读取
synchronized(sSoftBitmapCache){
SoftReference bitmapReference = sSoftBtimapCache.get(key);
if(bitmapReference != null){
final Bitmap bitmap2 = bitmapReference.get();
if(bitmap2 != null)
return bitmap2;
else{
Log.v("tag", "soft reference 已经被回收");
sSoftBitmapCache.remove(key);
}
}
}
return null;
}
2.外部文件缓存
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private File mCacheDir = context.getCacheDir();
private static final int MAX_CACHE_SIZE = 20 * 1024 * 1024; //20M
private final LruCache sFileCache = new LruCache(MAX_CACHE_SIZE){
@Override
public int sizeOf(String key, Long value){
return value.intValue();
}
@Override
protected void entryRemoved(boolean evicted, String key, Long oldValue, Long newValue){
File file = getFile(key);
if(file != null)
file.delete();
}
}
private File getFile(String fileName) throws FileNotFoundException {
File file = new File(mCacheDir, fileName);
if(!file.exists() || !file.isFile())
throw new FileNotFoundException("文件不存在或有同名文件夹");
return file;
}
//缓存bitmap到外部存储
public boolean putBitmap(String key, Bitmap bitmap){
File file = getFile(key);
if(file != null){
Log.v("tag", "文件已经存在");
return true;
}
FileOutputStream fos = getOutputStream(key);
boolean saved = bitmap.compress(CompressFormat.JPEG, 100, fos);
fos.flush();
fos.close();
if(saved){
synchronized(sFileCache){
sFileCache.put(key, getFile(key).length());
}
return true;
}
return false;
}
//根据key获取OutputStream
private FileOutputStream getOutputStream(String key){
if(mCacheDir == null)
return null;
FileOutputStream fos = new FileOutputStream(mCacheDir.getAbsolutePath() + File.separator + key);
return fos;
}
//获取bitmap
private static BitmapFactory.Options sBitmapOptions;
static {
sBitmapOptions = new BitmapFactory.Options();
sBitmapOptions.inPurgeable=true; //bitmap can be purged to disk
}
public Bitmap getBitmap(String key){
File bitmapFile = getFile(key);
if(bitmapFile != null){
Bitmap bitmap = BitmapFactory.decodeStream(new FileInputStream(bitmapFile), null, sBitmapOptions);
if(bitmap != null){
//重新将其缓存至硬引用中
...
}
}
}
3.从服务端下载图片
下载成功后调用1内存缓存的putBitmap()函数,缓存图片。
在外部文件缓存中也写入一份，调用2的putBitmap()函数.
4.预览图片的流程
1) 如果预览的图片在内存缓存区中，直接调用1的getBitmap()函数，获取bitmap数据(先在硬引用缓存区查找匹配，若硬引用区匹配失败，再去软引用区匹配)
2) 如果从内存缓存区读取失败，再从外部文件缓存中读取，调用2的getBitmap()函数
3) 如果从外部文件缓存中读取失败，则从服务端下载该图片，过程3.
5.生成key值
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private static String generateKey(String fileId, int width, int height) {
String ret = fileId + "_" + Integer.toString(width) + "x" + Integer.toString(height);
return ret;
}
String key = generateKey(...)即可生成唯一的key值
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